1. Field of the Invention
The invention relates to a print data processing apparatus that reduces the amount of dot image data by irreversible compression if the storage capacity for storing developed printable dot image data that is received is insufficient. The invention also relates to an input data generating apparatus that converts original image data generated into binary image data, and transmits the binary image data to the print data processing apparatus.
2. Description of the Related Art
Conventional print systems include a document processing apparatus, such as a host computer, a personal computer or similar apparatus which generates image data, and converts the image data into binary image print data, and transmits the print data to a page printer capable of high-speed printing, such as a laser printer.
In such a document processing apparatus, various application programs and a printer driver are installed. A color image or black-and-white character data, draw data or a graphic data is generated by an appropriate application program, and a printer driver performs binarization of such generated data. Specifically, the printer driver converts the generated data into binary image print data described in dot data of "1" or "0" using a page-description language (PDL), such as PostScript.RTM., and a print control language, such as Printer Control Language (PCL).RTM., and then transmits the binarized dot image data to a printer.
For the binarization, dithering is normally employed in which "1" or "0" is determined for each dot using a dither matrix. Other methods have also been put into practical use, such as an average error minimum method and an error diffusion method. In the average error minimum method and the error diffusion method, binarization is performed while an error between a specific pixel and an adjacent pixel is being evaluated on the basis of an error matrix. Regarding the dither matrix and the error matrix, a matrix is normally used that achieves an increased resolution by sacrificing gradation for a line image, or a matrix that achieves an increased gradation for a picture image.
As laser printers are generally termed "page printers", a typical laser printer prints an image for one page every time the printer receives print data for a page. A laser printer is normally equipped with a memory of several megabytes (MB) since a memory capacity of approximately 4 MB is required for dot image data for a page of, for example, A4 size, with a resolution of 300 dots per inch (DPI), and more memory capacity is required for storing font data. However, a memory shortage may occur when a large amount of dot image data for a page is to be developed. In such a case, either only a portion of the page data is printed, or an error occurs which prevents printing altogether. To solve this memory shortage problem, the invention disclosed in, for example, U.S. Pat. No. 5,604,846, reduces the amount of dot image data by performing a data thinning operation. Specifically, data is irreversibly compressed in such a manner that the resolution of the dot image data decreases.
Regarding the thinning operation, a simple thinning operation has been put into practical use which reduces the amount of dot data developed into the form of a matrix by selectively deleting dot data of, for example, all the even-numbered (or odd-numbered) dot rows and all the even-numbered (or odd-numbered) dot columns. An error evaluation thinning operation is proposed in, for example, Japanese Patent Application Laid-Open No. Hei 1-122267, which, when reducing the amount of dot data, performs evaluation with reference to error values based on the average density of a predetermined area of the original image data or arrangement conditions of dot data.
In page printers, such as laser printers, wherein the simple thinning of dot image data is always performed when low memory occurs, high speed thinning can be performed by simply deleting the data of a plurality of even-numbered (or odd-numbered) dot rows and the data of a plurality of even-numbered (or odd-numbered) dot columns. Although the simple thinning operation does not cause a problem in draft printing, or character or symbol printing, the simple thinning operation causes problems in images, such as photo-images, wherein density tone is important, or pop art-type line drawing images that have a regularity of dot arrangement. Thus, the density tone changes, or horizontal or vertical lines are partially lost, thereby degrading image quality.
The aforementioned error evaluation thinning operation may be used to solve these problems. However, if the error evaluation thinning of dot image data is always performed when low memory occurs, substantial deterioration of image quality (resolution) of photo-images or drawing images may be prevented but a long processing time is required due to the evaluation of error values based on dot arrangement. This is inconvenient for draft printing or character or symbol printing where a certain amount of image quality deterioration is acceptable.